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RFC3561 - Ad hoc On-Demand Distance Vector (AODV) Routing

王朝other·作者佚名  2008-05-31
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Network Working Group C. Perkins

Request for Comments: 3561 Nokia Research Center

Category: EXPerimental E. Belding-Royer

University of California, Santa Barbara

S. Das

University of Cincinnati

July 2003

Ad hoc On-Demand Distance Vector (AODV) Routing

Status of this Memo

This memo defines an Experimental Protocol for the Internet

community. It does not specify an Internet standard of any kind.

Discussion and suggestions for improvement are requested.

Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

The Ad hoc On-Demand Distance Vector (AODV) routing protocol is

intended for use by mobile nodes in an ad hoc network. It offers

quick adaptation to dynamic link conditions, low processing and

memory overhead, low network utilization, and determines unicast

routes to destinations within the ad hoc network. It uses

destination sequence numbers to ensure loop freedom at all times

(even in the face of anomalous delivery of routing control messages),

avoiding problems (sUCh as "counting to infinity") associated with

classical distance vector protocols.

Table of Contents

1. Introduction ............................................... 2

2. Overview .................................................. 3

3. AODV Terminology ........................................... 4

4. Applicability Statement .................................... 6

5. Message Formats ............................................ 7

5.1. Route Request (RREQ) Message Format ................... 7

5.2. Route Reply (RREP) Message Format ..................... 8

5.3. Route Error (RERR) Message Format ..................... 10

5.4. Route Reply Acknowledgment (RREP-ACK) Message Format .. 11

6. AODV Operation ............................................. 11

6.1. Maintaining Sequence Numbers .......................... 11

6.2. Route Table Entries and Precursor Lists ............... 13

6.3. Generating Route Requests ............................. 14

6.4. Controlling Dissemination of Route Request Messages ... 15

6.5. Processing and Forwarding Route Requests .............. 16

6.6. Generating Route Replies .............................. 18

6.6.1. Route Reply Generation by the Destination ...... 18

6.6.2. Route Reply Generation by an Intermediate

Node ........................................... 19

6.6.3. Generating Gratuitous RREPs .................... 19

6.7. Receiving and Forwarding Route Replies ................ 20

6.8. Operation over Unidirectional Links ................... 21

6.9. Hello Messages ........................................ 22

6.10 Maintaining Local Connectivity ........................ 23

6.11 Route Error (RERR) Messages, Route Expiry and Route

Deletion .............................................. 24

6.12 Local Repair .......................................... 26

6.13 Actions After Reboot ................................. 27

6.14 Interfaces ............................................ 28

7. AODV and Aggregated Networks ............................... 28

8. Using AODV with Other Networks ............................. 29

9. Extensions ................................................. 30

9.1. Hello Interval Extension Format ....................... 30

10. Configuration Parameters ................................... 31

11. Security Considerations .................................... 33

12. IANA Considerations ........................................ 34

13. IPv6 Considerations ........................................ 34

14. Acknowledgments ............................................ 34

15. Normative References ....................................... 35

16. Informative References ..................................... 35

17. Authors' Addresses ......................................... 36

18. Full Copyright Statement ................................... 37

1. Introduction

The Ad hoc On-Demand Distance Vector (AODV) algorithm enables

dynamic, self-starting, multihop routing between participating mobile

nodes wishing to establish and maintain an ad hoc network. AODV

allows mobile nodes to oBTain routes quickly for new destinations,

and does not require nodes to maintain routes to destinations that

are not in active communication. AODV allows mobile nodes to respond

to link breakages and changes in network topology in a timely manner.

The operation of AODV is loop-free, and by avoiding the Bellman-Ford

"counting to infinity" problem offers quick convergence when the ad

hoc network topology changes (typically, when a node moves in the

network). When links break, AODV causes the affected set of nodes to

be notified so that they are able to invalidate the routes using the

lost link.

One distinguishing feature of AODV is its use of a destination

sequence number for each route entry. The destination sequence

number is created by the destination to be included along with any

route information it sends to requesting nodes. Using destination

sequence numbers ensures loop freedom and is simple to program.

Given the choice between two routes to a destination, a requesting

node is required to select the one with the greatest sequence number.

2. Overview

Route Requests (RREQs), Route Replies (RREPs), and Route Errors

(RERRs) are the message types defined by AODV. These message types

are received via UDP, and normal IP header processing applies. So,

for instance, the requesting node is expected to use its IP address

as the Originator IP address for the messages. For broadcast

messages, the IP limited broadcast address (255.255.255.255) is used.

This means that such messages are not blindly forwarded. However,

AODV operation does require certain messages (e.g., RREQ) to be

disseminated widely, perhaps throughout the ad hoc network. The

range of dissemination of such RREQs is indicated by the TTL in the

IP header. Fragmentation is typically not required.

As long as the endpoints of a communication connection have valid

routes to each other, AODV does not play any role. When a route to a

new destination is needed, the node broadcasts a RREQ to find a route

to the destination. A route can be determined when the RREQ reaches

either the destination itself, or an intermediate node with a 'fresh

enough' route to the destination. A 'fresh enough' route is a valid

route entry for the destination whose associated sequence number is

at least as great as that contained in the RREQ. The route is made

available by unicasting a RREP back to the origination of the RREQ.

Each node receiving the request caches a route back to the originator

of the request, so that the RREP can be unicast from the destination

along a path to that originator, or likewise from any intermediate

node that is able to satisfy the request.

Nodes monitor the link status of next hops in active routes. When a

link break in an active route is detected, a RERR message is used to

notify other nodes that the loss of that link has occurred. The RERR

message indicates those destinations (possibly subnets) which are no

longer reachable by way of the broken link. In order to enable this

reporting mechanism, each node keeps a "precursor list", containing

the IP address for each its neighbors that are likely to use it as a

next hop towards each destination. The information in the precursor

lists is most easily acquired during the processing for generation of

a RREP message, which by definition has to be sent to a node in a

precursor list (see section 6.6). If the RREP has a nonzero prefix

length, then the originator of the RREQ which solicited the RREP

information is included among the precursors for the subnet route

(not specifically for the particular destination).

A RREQ may also be received for a multicast IP address. In this

document, full processing for such messages is not specified. For

example, the originator of such a RREQ for a multicast IP address may

have to follow special rules. However, it is important to enable

correct multicast operation by intermediate nodes that are not

enabled as originating or destination nodes for IP multicast

addresses, and likewise are not equipped for any special multicast

protocol processing. For such multicast-unaware nodes, processing

for a multicast IP address as a destination IP address MUST be

carried out in the same way as for any other destination IP address.

AODV is a routing protocol, and it deals with route table management.

Route table information must be kept even for short-lived routes,

such as are created to temporarily store reverse paths towards nodes

originating RREQs. AODV uses the following fields with each route

table entry:

- Destination IP Address

- Destination Sequence Number

- Valid Destination Sequence Number flag

- Other state and routing flags (e.g., valid, invalid, repairable,

being repaired)

- Network Interface

- Hop Count (number of hops needed to reach destination)

- Next Hop

- List of Precursors (described in Section 6.2)

- Lifetime (expiration or deletion time of the route)

Managing the sequence number is crucial to avoiding routing loops,

even when links break and a node is no longer reachable to supply its

own information about its sequence number. A destination becomes

unreachable when a link breaks or is deactivated. When these

conditions occur, the route is invalidated by operations involving

the sequence number and marking the route table entry state as

invalid. See section 6.1 for details.

3. AODV Terminology

This protocol specification uses conventional meanings [1] for

capitalized Words such as MUST, SHOULD, etc., to indicate requirement

levels for various protocol features. This section defines other

terminology used with AODV that is not already defined in [3].

active route

A route towards a destination that has a routing table entry

that is marked as valid. Only active routes can be used to

forward data packets.

broadcast

Broadcasting means transmitting to the IP Limited Broadcast

address, 255.255.255.255. A broadcast packet may not be

blindly forwarded, but broadcasting is useful to enable

dissemination of AODV messages throughout the ad hoc network.

destination

An IP address to which data packets are to be transmitted.

Same as "destination node". A node knows it is the destination

node for a typical data packet when its address appears in the

appropriate field of the IP header. Routes for destination

nodes are supplied by action of the AODV protocol, which

carries the IP address of the desired destination node in route

discovery messages.

forwarding node

A node that agrees to forward packets destined for another

node, by retransmitting them to a next hop that is closer to

the unicast destination along a path that has been set up using

routing control messages.

forward route

A route set up to send data packets from a node originating a

Route Discovery operation towards its desired destination.

invalid route

A route that has expired, denoted by a state of invalid in the

routing table entry. An invalid route is used to store

previously valid route information for an extended period of

time. An invalid route cannot be used to forward data packets,

but it can provide information useful for route repairs, and

also for future RREQ messages.

originating node

A node that initiates an AODV route discovery message to be

processed and possibly retransmitted by other nodes in the ad

hoc network. For instance, the node initiating a Route

Discovery process and broadcasting the RREQ message is called

the originating node of the RREQ message.

reverse route

A route set up to forward a reply (RREP) packet back to the

originator from the destination or from an intermediate node

having a route to the destination.

sequence number

A monotonically increasing number maintained by each

originating node. In AODV routing protocol messages, it is

used by other nodes to determine the freshness of the

information contained from the originating node.

valid route

See active route.

4. Applicability Statement

The AODV routing protocol is designed for mobile ad hoc networks with

populations of tens to thousands of mobile nodes. AODV can handle

low, moderate, and relatively high mobility rates, as well as a

variety of data traffic levels. AODV is designed for use in networks

where the nodes can all trust each other, either by use of

preconfigured keys, or because it is known that there are no

malicious intruder nodes. AODV has been designed to reduce the

dissemination of control traffic and eliminate overhead on data

traffic, in order to improve scalability and performance.

5. Message Formats

5.1. Route Request (RREQ) Message Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type JRGDU Reserved Hop Count

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

RREQ ID

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination IP Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination Sequence Number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Originator IP Address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Originator Sequence Number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the Route Request message is illustrated above, and

contains the following fields:

Type 1

J Join flag; reserved for multicast.

R Repair flag; reserved for multicast.

G Gratuitous RREP flag; indicates whether a

gratuitous RREP should be unicast to the node

specified in the Destination IP Address field (see

sections 6.3, 6.6.3).

D Destination only flag; indicates only the

destination may respond to this RREQ (see

section 6.5).

U Unknown sequence number; indicates the destination

sequence number is unknown (see section 6.3).

Reserved Sent as 0; ignored on reception.

Hop Count The number of hops from the Originator IP Address

to the node handling the request.

RREQ ID A sequence number uniquely identifying the

particular RREQ when taken in conjunction with the

originating node's IP address.

Destination IP Address

The IP address of the destination for which a route

is desired.

Destination Sequence Number

The latest sequence number received in the past

by the originator for any route towards the

destination.

Originator IP Address

The IP address of the node which originated the

Route Request.

Originator Sequence Number

The current sequence number to be used in the route

entry pointing towards the originator of the route

request.

5.2. Route Reply (RREP) Message Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type RA Reserved Prefix Sz Hop Count

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination IP address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Destination Sequence Number

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Originator IP address

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Lifetime

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the Route Reply message is illustrated above, and

contains the following fields:

Type 2

R Repair flag; used for multicast.

A Acknowledgment required; see sections 5.4 and 6.7.

Reserved Sent as 0; ignored on reception.

Prefix Size If nonzero, the 5-bit Prefix Size specifies that the

indicated next hop may be used for any nodes with

the same routing prefix (as defined by the Prefix

Size) as the requested destination.

Hop Count The number of hops from the Originator IP Address

to the Destination IP Address. For multicast route

requests this indicates the number of hops to the

multicast tree member sending the RREP.

Destination IP Address

The IP address of the destination for which a route

is supplied.

Destination Sequence Number

The destination sequence number associated to the

route.

Originator IP Address

The IP address of the node which originated the RREQ

for which the route is supplied.

Lifetime The time in milliseconds for which nodes receiving

the RREP consider the route to be valid.

Note that the Prefix Size allows a subnet router to supply a route

for every host in the subnet defined by the routing prefix, which is

determined by the IP address of the subnet router and the Prefix

Size. In order to make use of this feature, the subnet router has to

guarantee reachability to all the hosts sharing the indicated subnet

prefix. See section 7 for details. When the prefix size is nonzero,

any routing information (and precursor data) MUST be kept with

respect to the subnet route, not the individual destination IP

address on that subnet.

The 'A' bit is used when the link over which the RREP message is sent

may be unreliable or unidirectional. When the RREP message contains

the 'A' bit set, the receiver of the RREP is expected to return a

RREP-ACK message. See section 6.8.

5.3. Route Error (RERR) Message Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type N Reserved DestCount

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Unreachable Destination IP Address (1)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Unreachable Destination Sequence Number (1)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

Additional Unreachable Destination IP Addresses (if needed)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Additional Unreachable Destination Sequence Numbers (if needed)

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The format of the Route Error message is illustrated above, and

contains the following fields:

Type 3

N No delete flag; set when a node has performed a local

repair of a link, and upstream nodes should not delete

the route.

Reserved Sent as 0; ignored on reception.

DestCount The number of unreachable destinations included in the

message; MUST be at least 1.

Unreachable Destination IP Address

The IP address of the destination that has become

unreachable due to a link break.

Unreachable Destination Sequence Number

The sequence number in the route table entry for

the destination listed in the previous Unreachable

Destination IP Address field.

The RERR message is sent whenever a link break causes one or more

destinations to become unreachable from some of the node's neighbors.

See section 6.2 for information about how to maintain the appropriate

records for this determination, and section 6.11 for specification

about how to create the list of destinations.

5.4. Route Reply Acknowledgment (RREP-ACK) Message Format

The Route Reply Acknowledgment (RREP-ACK) message MUST be sent in

response to a RREP message with the 'A' bit set (see section 5.2).

This is typically done when there is danger of unidirectional links

preventing the completion of a Route Discovery cycle (see section

6.8).

0 1

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Reserved

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 4

Reserved Sent as 0; ignored on reception.

6. AODV Operation

This section describes the scenarios under which nodes generate Route

Request (RREQ), Route Reply (RREP) and Route Error (RERR) messages

for unicast communication towards a destination, and how the message

data are handled. In order to process the messages correctly,

certain state information has to be maintained in the route table

entries for the destinations of interest.

All AODV messages are sent to port 654 using UDP.

6.1. Maintaining Sequence Numbers

Every route table entry at every node MUST include the latest

information available about the sequence number for the IP address of

the destination node for which the route table entry is maintained.

This sequence number is called the "destination sequence number". It

is updated whenever a node receives new (i.e., not stale) information

about the sequence number from RREQ, RREP, or RERR messages that may

be received related to that destination. AODV depends on each node

in the network to own and maintain its destination sequence number to

guarantee the loop-freedom of all routes towards that node. A

destination node increments its own sequence number in two

circumstances:

- Immediately before a node originates a route discovery, it MUST

increment its own sequence number. This prevents conflicts with

previously established reverse routes towards the originator of a

RREQ.

- Immediately before a destination node originates a RREP in

response to a RREQ, it MUST update its own sequence number to the

maximum of its current sequence number and the destination

sequence number in the RREQ packet.

When the destination increments its sequence number, it MUST do so by

treating the sequence number value as if it were an unsigned number.

To accomplish sequence number rollover, if the sequence number has

already been assigned to be the largest possible number representable

as a 32-bit unsigned integer (i.e., 4294967295), then when it is

incremented it will then have a value of zero (0). On the other

hand, if the sequence number currently has the value 2147483647,

which is the largest possible positive integer if 2's complement

arithmetic is in use with 32-bit integers, the next value will be

2147483648, which is the most negative possible integer in the same

numbering system. The representation of negative numbers is not

relevant to the increment of AODV sequence numbers. This is in

contrast to the manner in which the result of comparing two AODV

sequence numbers is to be treated (see below).

In order to ascertain that information about a destination is not

stale, the node compares its current numerical value for the sequence

number with that obtained from the incoming AODV message. This

comparison MUST be done using signed 32-bit arithmetic, this is

necessary to accomplish sequence number rollover. If the result of

subtracting the currently stored sequence number from the value of

the incoming sequence number is less than zero, then the information

related to that destination in the AODV message MUST be discarded,

since that information is stale compared to the node's currently

stored information.

The only other circumstance in which a node may change the

destination sequence number in one of its route table entries is in

response to a lost or expired link to the next hop towards that

destination. The node determines which destinations use a particular

next hop by consulting its routing table. In this case, for each

destination that uses the next hop, the node increments the sequence

number and marks the route as invalid (see also sections 6.11, 6.12).

Whenever any fresh enough (i.e., containing a sequence number at

least equal to the recorded sequence number) routing information for

an affected destination is received by a node that has marked that

route table entry as invalid, the node SHOULD update its route table

information according to the information contained in the update.

A node may change the sequence number in the routing table entry of a

destination only if:

- it is itself the destination node, and offers a new route to

itself, or

- it receives an AODV message with new information about the

sequence number for a destination node, or

- the path towards the destination node expires or breaks.

6.2. Route Table Entries and Precursor Lists

When a node receives an AODV control packet from a neighbor, or

creates or updates a route for a particular destination or subnet, it

checks its route table for an entry for the destination. In the

event that there is no corresponding entry for that destination, an

entry is created. The sequence number is either determined from the

information contained in the control packet, or else the valid

sequence number field is set to false. The route is only updated if

the new sequence number is either

(i) higher than the destination sequence number in the route

table, or

(ii) the sequence numbers are equal, but the hop count (of the

new information) plus one, is smaller than the existing hop

count in the routing table, or

(iii) the sequence number is unknown.

The Lifetime field of the routing table entry is either determined

from the control packet, or it is initialized to

ACTIVE_ROUTE_TIMEOUT. This route may now be used to send any queued

data packets and fulfills any outstanding route requests.

Each time a route is used to forward a data packet, its Active Route

Lifetime field of the source, destination and the next hop on the

path to the destination is updated to be no less than the current

time plus ACTIVE_ROUTE_TIMEOUT. Since the route between each

originator and destination pair is expected to be symmetric, the

Active Route Lifetime for the previous hop, along the reverse path

back to the IP source, is also updated to be no less than the current

time plus ACTIVE_ROUTE_TIMEOUT. The lifetime for an Active Route is

updated each time the route is used regardless of whether the

destination is a single node or a subnet.

For each valid route maintained by a node as a routing table entry,

the node also maintains a list of precursors that may be forwarding

packets on this route. These precursors will receive notifications

from the node in the event of detection of the loss of the next hop

link. The list of precursors in a routing table entry contains those

neighboring nodes to which a route reply was generated or forwarded.

6.3. Generating Route Requests

A node disseminates a RREQ when it determines that it needs a route

to a destination and does not have one available. This can happen if

the destination is previously unknown to the node, or if a previously

valid route to the destination expires or is marked as invalid. The

Destination Sequence Number field in the RREQ message is the last

known destination sequence number for this destination and is copied

from the Destination Sequence Number field in the routing table. If

no sequence number is known, the unknown sequence number flag MUST be

set. The Originator Sequence Number in the RREQ message is the

node's own sequence number, which is incremented prior to insertion

in a RREQ. The RREQ ID field is incremented by one from the last

RREQ ID used by the current node. Each node maintains only one RREQ

ID. The Hop Count field is set to zero.

Before broadcasting the RREQ, the originating node buffers the RREQ

ID and the Originator IP address (its own address) of the RREQ for

PATH_DISCOVERY_TIME. In this way, when the node receives the packet

again from its neighbors, it will not reprocess and re-forward the

packet.

An originating node often expects to have bidirectional

communications with a destination node. In such cases, it is not

sufficient for the originating node to have a route to the

destination node; the destination must also have a route back to the

originating node. In order for this to happen as efficiently as

possible, any generation of a RREP by an intermediate node (as in

section 6.6) for delivery to the originating node SHOULD be

accompanied by some action that notifies the destination about a

route back to the originating node. The originating node selects

this mode of operation in the intermediate nodes by setting the 'G'

flag. See section 6.6.3 for details about actions taken by the

intermediate node in response to a RREQ with the 'G' flag set.

A node SHOULD NOT originate more than RREQ_RATELIMIT RREQ messages

per second. After broadcasting a RREQ, a node waits for a RREP (or

other control message with current information regarding a route to

the appropriate destination). If a route is not received within

NET_TRAVERSAL_TIME milliseconds, the node MAY try again to discover a

route by broadcasting another RREQ, up to a maximum of RREQ_RETRIES

times at the maximum TTL value. Each new attempt MUST increment and

update the RREQ ID. For each attempt, the TTL field of the IP header

is set according to the mechanism specified in section 6.4, in order

to enable control over how far the RREQ is disseminated for the each

retry.

Data packets waiting for a route (i.e., waiting for a RREP after a

RREQ has been sent) SHOULD be buffered. The buffering SHOULD be

"first-in, first-out" (FIFO). If a route discovery has been

attempted RREQ_RETRIES times at the maximum TTL without receiving any

RREP, all data packets destined for the corresponding destination

SHOULD be dropped from the buffer and a Destination Unreachable

message SHOULD be delivered to the application.

To reduce congestion in a network, repeated attempts by a source node

at route discovery for a single destination MUST utilize a binary

exponential bacKOFf. The first time a source node broadcasts a RREQ,

it waits NET_TRAVERSAL_TIME milliseconds for the reception of a RREP.

If a RREP is not received within that time, the source node sends a

new RREQ. When calculating the time to wait for the RREP after

sending the second RREQ, the source node MUST use a binary

exponential backoff. Hence, the waiting time for the RREP

corresponding to the second RREQ is 2 * NET_TRAVERSAL_TIME

milliseconds. If a RREP is not received within this time period,

another RREQ may be sent, up to RREQ_RETRIES additional attempts

after the first RREQ. For each additional attempt, the waiting time

for the RREP is multiplied by 2, so that the time conforms to a

binary exponential backoff.

6.4. Controlling Dissemination of Route Request Messages

To prevent unnecessary network-wide dissemination of RREQs, the

originating node SHOULD use an expanding ring search technique. In

an expanding ring search, the originating node initially uses a TTL =

TTL_START in the RREQ packet IP header and sets the timeout for

receiving a RREP to RING_TRAVERSAL_TIME milliseconds.

RING_TRAVERSAL_TIME is calculated as described in section 10. The

TTL_VALUE used in calculating RING_TRAVERSAL_TIME is set equal to the

value of the TTL field in the IP header. If the RREQ times out

without a corresponding RREP, the originator broadcasts the RREQ

again with the TTL incremented by TTL_INCREMENT. This continues

until the TTL set in the RREQ reaches TTL_THRESHOLD, beyond which a

TTL = NET_DIAMETER is used for each attempt. Each time, the timeout

for receiving a RREP is RING_TRAVERSAL_TIME. When it is desired to

have all retries traverse the entire ad hoc network, this can be

achieved by configuring TTL_START and TTL_INCREMENT both to be the

same value as NET_DIAMETER.

The Hop Count stored in an invalid routing table entry indicates the

last known hop count to that destination in the routing table. When

a new route to the same destination is required at a later time

(e.g., upon route loss), the TTL in the RREQ IP header is initially

set to the Hop Count plus TTL_INCREMENT. Thereafter, following each

timeout the TTL is incremented by TTL_INCREMENT until TTL =

TTL_THRESHOLD is reached. Beyond this TTL = NET_DIAMETER is used.

Once TTL = NET_DIAMETER, the timeout for waiting for the RREP is set

to NET_TRAVERSAL_TIME, as specified in section 6.3.

An expired routing table entry SHOULD NOT be expunged before

(current_time + DELETE_PERIOD) (see section 6.11). Otherwise, the

soft state corresponding to the route (e.g., last known hop count)

will be lost. Furthermore, a longer routing table entry expunge time

MAY be configured. Any routing table entry waiting for a RREP SHOULD

NOT be expunged before (current_time + 2 * NET_TRAVERSAL_TIME).

6.5. Processing and Forwarding Route Requests

When a node receives a RREQ, it first creates or updates a route to

the previous hop without a valid sequence number (see section 6.2)

then checks to determine whether it has received a RREQ with the same

Originator IP Address and RREQ ID within at least the last

PATH_DISCOVERY_TIME. If such a RREQ has been received, the node

silently discards the newly received RREQ. The rest of this

subsection describes actions taken for RREQs that are not discarded.

First, it first increments the hop count value in the RREQ by one, to

account for the new hop through the intermediate node. Then the node

searches for a reverse route to the Originator IP Address (see

section 6.2), using longest-prefix matching. If need be, the route

is created, or updated using the Originator Sequence Number from the

RREQ in its routing table. This reverse route will be needed if the

node receives a RREP back to the node that originated the RREQ

(identified by the Originator IP Address). When the reverse route is

created or updated, the following actions on the route are also

carried out:

1. the Originator Sequence Number from the RREQ is compared to the

corresponding destination sequence number in the route table entry

and copied if greater than the existing value there

2. the valid sequence number field is set to true;

3. the next hop in the routing table becomes the node from which the

RREQ was received (it is obtained from the source IP address in

the IP header and is often not equal to the Originator IP Address

field in the RREQ message);

4. the hop count is copied from the Hop Count in the RREQ message;

Whenever a RREQ message is received, the Lifetime of the reverse

route entry for the Originator IP address is set to be the maximum of

(ExistingLifetime, MinimalLifetime), where

MinimalLifetime = (current time + 2*NET_TRAVERSAL_TIME -

2*HopCount*NODE_TRAVERSAL_TIME).

The current node can use the reverse route to forward data packets in

the same way as for any other route in the routing table.

If a node does not generate a RREP (following the processing rules in

section 6.6), and if the incoming IP header has TTL larger than 1,

the node updates and broadcasts the RREQ to address 255.255.255.255

on each of its configured interfaces (see section 6.14). To update

the RREQ, the TTL or hop limit field in the outgoing IP header is

decreased by one, and the Hop Count field in the RREQ message is

incremented by one, to account for the new hop through the

intermediate node. Lastly, the Destination Sequence number for the

requested destination is set to the maximum of the corresponding

value received in the RREQ message, and the destination sequence

value currently maintained by the node for the requested destination.

However, the forwarding node MUST NOT modify its maintained value for

the destination sequence number, even if the value received in the

incoming RREQ is larger than the value currently maintained by the

forwarding node.

Otherwise, if a node does generate a RREP, then the node discards the

RREQ. Notice that, if intermediate nodes reply to every transmission

of RREQs for a particular destination, it might turn out that the

destination does not receive any of the discovery messages. In this

situation, the destination does not learn of a route to the

originating node from the RREQ messages. This could cause the

destination to initiate a route discovery (for example, if the

originator is attempting to establish a TCP session). In order that

the destination learn of routes to the originating node, the

originating node SHOULD set the "gratuitous RREP" ('G') flag in the

RREQ if for any reason the destination is likely to need a route to

the originating node. If, in response to a RREQ with the 'G' flag

set, an intermediate node returns a RREP, it MUST also unicast a

gratuitous RREP to the destination node (see section 6.6.3).

6.6. Generating Route Replies

A node generates a RREP if either:

(i) it is itself the destination, or

(ii) it has an active route to the destination, the destination

sequence number in the node's existing route table entry

for the destination is valid and greater than or equal to

the Destination Sequence Number of the RREQ (comparison

using signed 32-bit arithmetic), and the "destination only"

('D') flag is NOT set.

When generating a RREP message, a node copies the Destination IP

Address and the Originator Sequence Number from the RREQ message into

the corresponding fields in the RREP message. Processing is slightly

different, depending on whether the node is itself the requested

destination (see section 6.6.1), or instead if it is an intermediate

node with an fresh enough route to the destination (see section

6.6.2).

Once created, the RREP is unicast to the next hop toward the

originator of the RREQ, as indicated by the route table entry for

that originator. As the RREP is forwarded back towards the node

which originated the RREQ message, the Hop Count field is incremented

by one at each hop. Thus, when the RREP reaches the originator, the

Hop Count represents the distance, in hops, of the destination from

the originator.

6.6.1. Route Reply Generation by the Destination

If the generating node is the destination itself, it MUST increment

its own sequence number by one if the sequence number in the RREQ

packet is equal to that incremented value. Otherwise, the

destination does not change its sequence number before generating the

RREP message. The destination node places its (perhaps newly

incremented) sequence number into the Destination Sequence Number

field of the RREP, and enters the value zero in the Hop Count field

of the RREP.

The destination node copies the value MY_ROUTE_TIMEOUT (see section

10) into the Lifetime field of the RREP. Each node MAY reconfigure

its value for MY_ROUTE_TIMEOUT, within mild constraints (see section

10).

6.6.2. Route Reply Generation by an Intermediate Node

If the node generating the RREP is not the destination node, but

instead is an intermediate hop along the path from the originator to

the destination, it copies its known sequence number for the

destination into the Destination Sequence Number field in the RREP

message.

The intermediate node updates the forward route entry by placing the

last hop node (from which it received the RREQ, as indicated by the

source IP address field in the IP header) into the precursor list for

the forward route entry -- i.e., the entry for the Destination IP

Address. The intermediate node also updates its route table entry

for the node originating the RREQ by placing the next hop towards the

destination in the precursor list for the reverse route entry --

i.e., the entry for the Originator IP Address field of the RREQ

message data.

The intermediate node places its distance in hops from the

destination (indicated by the hop count in the routing table) Count

field in the RREP. The Lifetime field of the RREP is calculated by

subtracting the current time from the expiration time in its route

table entry.

6.6.3. Generating Gratuitous RREPs

After a node receives a RREQ and responds with a RREP, it discards

the RREQ. If the RREQ has the 'G' flag set, and the intermediate

node returns a RREP to the originating node, it MUST also unicast a

gratuitous RREP to the destination node. The gratuitous RREP that is

to be sent to the desired destination contains the following values

in the RREP message fields:

Hop Count The Hop Count as indicated in the

node's route table entry for the

originator

Destination IP Address The IP address of the node that

originated the RREQ

Destination Sequence Number The Originator Sequence Number from

the RREQ

Originator IP Address The IP address of the Destination

node in the RREQ

Lifetime The remaining lifetime of the route

towards the originator of the RREQ,

as known by the intermediate node.

The gratuitous RREP is then sent to the next hop along the path to

the destination node, just as if the destination node had already

issued a RREQ for the originating node and this RREP was produced in

response to that (fictitious) RREQ. The RREP that is sent to the

originator of the RREQ is the same whether or not the 'G' bit is set.

6.7. Receiving and Forwarding Route Replies

When a node receives a RREP message, it searches (using longest-

prefix matching) for a route to the previous hop. If needed, a route

is created for the previous hop, but without a valid sequence number

(see section 6.2). Next, the node then increments the hop count

value in the RREP by one, to account for the new hop through the

intermediate node. Call this incremented value the "New Hop Count".

Then the forward route for this destination is created if it does not

already exist. Otherwise, the node compares the Destination Sequence

Number in the message with its own stored destination sequence number

for the Destination IP Address in the RREP message. Upon comparison,

the existing entry is updated only in the following circumstances:

(i) the sequence number in the routing table is marked as

invalid in route table entry.

(ii) the Destination Sequence Number in the RREP is greater than

the node's copy of the destination sequence number and the

known value is valid, or

(iii) the sequence numbers are the same, but the route is is

marked as inactive, or

(iv) the sequence numbers are the same, and the New Hop Count is

smaller than the hop count in route table entry.

If the route table entry to the destination is created or updated,

then the following actions occur:

- the route is marked as active,

- the destination sequence number is marked as valid,

- the next hop in the route entry is assigned to be the node from

which the RREP is received, which is indicated by the source IP

address field in the IP header,

- the hop count is set to the value of the New Hop Count,

- the expiry time is set to the current time plus the value of the

Lifetime in the RREP message,

- and the destination sequence number is the Destination Sequence

Number in the RREP message.

The current node can subsequently use this route to forward data

packets to the destination.

If the current node is not the node indicated by the Originator IP

Address in the RREP message AND a forward route has been created or

updated as described above, the node consults its route table entry

for the originating node to determine the next hop for the RREP

packet, and then forwards the RREP towards the originator using the

information in that route table entry. If a node forwards a RREP

over a link that is likely to have errors or be unidirectional, the

node SHOULD set the 'A' flag to require that the recipient of the

RREP acknowledge receipt of the RREP by sending a RREP-ACK message

back (see section 6.8).

When any node transmits a RREP, the precursor list for the

corresponding destination node is updated by adding to it the next

hop node to which the RREP is forwarded. Also, at each node the

(reverse) route used to forward a RREP has its lifetime changed to be

the maximum of (existing-lifetime, (current time +

ACTIVE_ROUTE_TIMEOUT). Finally, the precursor list for the next hop

towards the destination is updated to contain the next hop towards

the source.

6.8. Operation over Unidirectional Links

It is possible that a RREP transmission may fail, especially if the

RREQ transmission triggering the RREP occurs over a unidirectional

link. If no other RREP generated from the same route discovery

attempt reaches the node which originated the RREQ message, the

originator will reattempt route discovery after a timeout (see

section 6.3). However, the same scenario might well be repeated

without any improvement, and no route would be discovered even after

repeated retries. Unless corrective action is taken, this can happen

even when bidirectional routes between originator and destination do

exist. Link layers using broadcast transmissions for the RREQ will

not be able to detect the presence of such unidirectional links. In

AODV, any node acts on only the first RREQ with the same RREQ ID and

ignores any subsequent RREQs. Suppose, for example, that the first

RREQ arrives along a path that has one or more unidirectional

link(s). A subsequent RREQ may arrive via a bidirectional path

(assuming such paths exist), but it will be ignored.

To prevent this problem, when a node detects that its transmission of

a RREP message has failed, it remembers the next-hop of the failed

RREP in a "blacklist" set. Such failures can be detected via the

absence of a link-layer or network-layer acknowledgment (e.g., RREP-

ACK). A node ignores all RREQs received from any node in its

blacklist set. Nodes are removed from the blacklist set after a

BLACKLIST_TIMEOUT period (see section 10). This period should be set

to the upper bound of the time it takes to perform the allowed number

of route request retry attempts as described in section 6.3.

Note that the RREP-ACK packet does not contain any information about

which RREP it is acknowledging. The time at which the RREP-ACK is

received will likely come just after the time when the RREP was sent

with the 'A' bit. This information is expected to be sufficient to

provide assurance to the sender of the RREP that the link is

currently bidirectional, without any real dependence on the

particular RREP message being acknowledged. However, that assurance

typically cannot be expected to remain in force permanently.

6.9. Hello Messages

A node MAY offer connectivity information by broadcasting local Hello

messages. A node SHOULD only use hello messages if it is part of an

active route. Every HELLO_INTERVAL milliseconds, the node checks

whether it has sent a broadcast (e.g., a RREQ or an appropriate layer

2 message) within the last HELLO_INTERVAL. If it has not, it MAY

broadcast a RREP with TTL = 1, called a Hello message, with the RREP

message fields set as follows:

Destination IP Address The node's IP address.

Destination Sequence Number The node's latest sequence number.

Hop Count 0

Lifetime ALLOWED_HELLO_LOSS * HELLO_INTERVAL

A node MAY determine connectivity by listening for packets from its

set of neighbors. If, within the past DELETE_PERIOD, it has received

a Hello message from a neighbor, and then for that neighbor does not

receive any packets (Hello messages or otherwise) for more than

ALLOWED_HELLO_LOSS * HELLO_INTERVAL milliseconds, the node SHOULD

assume that the link to this neighbor is currently lost. When this

happens, the node SHOULD proceed as in Section 6.11.

Whenever a node receives a Hello message from a neighbor, the node

SHOULD make sure that it has an active route to the neighbor, and

create one if necessary. If a route already exists, then the

Lifetime for the route should be increased, if necessary, to be at

least ALLOWED_HELLO_LOSS * HELLO_INTERVAL. The route to the

neighbor, if it exists, MUST subsequently contain the latest

Destination Sequence Number from the Hello message. The current node

can now begin using this route to forward data packets. Routes that

are created by hello messages and not used by any other active routes

will have empty precursor lists and would not trigger a RERR message

if the neighbor moves away and a neighbor timeout occurs.

6.10. Maintaining Local Connectivity

Each forwarding node SHOULD keep track of its continued connectivity

to its active next hops (i.e., which next hops or precursors have

forwarded packets to or from the forwarding node during the last

ACTIVE_ROUTE_TIMEOUT), as well as neighbors that have transmitted

Hello messages during the last (ALLOWED_HELLO_LOSS * HELLO_INTERVAL).

A node can maintain accurate information about its continued

connectivity to these active next hops, using one or more of the

available link or network layer mechanisms, as described below.

- Any suitable link layer notification, such as those provided by

IEEE 802.11, can be used to determine connectivity, each time a

packet is transmitted to an active next hop. For example, absence

of a link layer ACK or failure to get a CTS after sending RTS,

even after the maximum number of retransmission attempts,

indicates loss of the link to this active next hop.

- If layer-2 notification is not available, passive acknowledgment

SHOULD be used when the next hop is expected to forward the

packet, by listening to the channel for a transmission attempt

made by the next hop. If transmission is not detected within

NEXT_HOP_WAIT milliseconds or the next hop is the destination (and

thus is not supposed to forward the packet) one of the following

methods SHOULD be used to determine connectivity:

* Receiving any packet (including a Hello message) from the next

hop.

* A RREQ unicast to the next hop, aSKINg for a route to the next

hop.

* An ICMP Echo Request message unicast to the next hop.

If a link to the next hop cannot be detected by any of these methods,

the forwarding node SHOULD assume that the link is lost, and take

corrective action by following the methods specified in Section 6.11.

6.11. Route Error (RERR) Messages, Route Expiry and Route Deletion

Generally, route error and link breakage processing requires the

following steps:

- Invalidating existing routes

- Listing affected destinations

- Determining which, if any, neighbors may be affected

- Delivering an appropriate RERR to such neighbors

A Route Error (RERR) message MAY be either broadcast (if there are

many precursors), unicast (if there is only 1 precursor), or

iteratively unicast to all precursors (if broadcast is

inappropriate). Even when the RERR message is iteratively unicast to

several precursors, it is considered to be a single control message

for the purposes of the description in the text that follows. With

that understanding, a node SHOULD NOT generate more than

RERR_RATELIMIT RERR messages per second.

A node initiates processing for a RERR message in three situations:

(i) if it detects a link break for the next hop of an active

route in its routing table while transmitting data (and

route repair, if attempted, was unsuccessful), or

(ii) if it gets a data packet destined to a node for which it

does not have an active route and is not repairing (if

using local repair), or

(iii) if it receives a RERR from a neighbor for one or more

active routes.

For case (i), the node first makes a list of unreachable destinations

consisting of the unreachable neighbor and any additional

destinations (or subnets, see section 7) in the local routing table

that use the unreachable neighbor as the next hop. In this case, if

a subnet route is found to be newly unreachable, an IP destination

address for the subnet is constructed by appending zeroes to the

subnet prefix as shown in the route table entry. This is

unambiguous, since the precursor is known to have route table

information with a compatible prefix length for that subnet.

For case (ii), there is only one unreachable destination, which is

the destination of the data packet that cannot be delivered. For

case (iii), the list should consist of those destinations in the RERR

for which there exists a corresponding entry in the local routing

table that has the transmitter of the received RERR as the next hop.

Some of the unreachable destinations in the list could be used by

neighboring nodes, and it may therefore be necessary to send a (new)

RERR. The RERR should contain those destinations that are part of

the created list of unreachable destinations and have a non-empty

precursor list.

The neighboring node(s) that should receive the RERR are all those

that belong to a precursor list of at least one of the unreachable

destination(s) in the newly created RERR. In case there is only one

unique neighbor that needs to receive the RERR, the RERR SHOULD be

unicast toward that neighbor. Otherwise the RERR is typically sent

to the local broadcast address (Destination IP == 255.255.255.255,

TTL == 1) with the unreachable destinations, and their corresponding

destination sequence numbers, included in the packet. The DestCount

field of the RERR packet indicates the number of unreachable

destinations included in the packet.

Just before transmitting the RERR, certain updates are made on the

routing table that may affect the destination sequence numbers for

the unreachable destinations. For each one of these destinations,

the corresponding routing table entry is updated as follows:

1. The destination sequence number of this routing entry, if it

exists and is valid, is incremented for cases (i) and (ii) above,

and copied from the incoming RERR in case (iii) above.

2. The entry is invalidated by marking the route entry as invalid

3. The Lifetime field is updated to current time plus DELETE_PERIOD.

Before this time, the entry SHOULD NOT be deleted.

Note that the Lifetime field in the routing table plays dual role --

for an active route it is the expiry time, and for an invalid route

it is the deletion time. If a data packet is received for an invalid

route, the Lifetime field is updated to current time plus

DELETE_PERIOD. The determination of DELETE_PERIOD is discussed in

Section 10.

6.12. Local Repair

When a link break in an active route occurs, the node upstream of

that break MAY choose to repair the link locally if the destination

was no farther than MAX_REPAIR_TTL hops away. To repair the link

break, the node increments the sequence number for the destination

and then broadcasts a RREQ for that destination. The TTL of the RREQ

should initially be set to the following value:

max(MIN_REPAIR_TTL, 0.5 * #hops) + LOCAL_ADD_TTL,

where #hops is the number of hops to the sender (originator) of the

currently undeliverable packet. Thus, local repair attempts will

often be invisible to the originating node, and will always have TTL

>= MIN_REPAIR_TTL + LOCAL_ADD_TTL. The node initiating the repair

then waits the discovery period to receive RREPs in response to the

RREQ. During local repair data packets SHOULD be buffered. If, at

the end of the discovery period, the repairing node has not received

a RREP (or other control message creating or updating the route) for

that destination, it proceeds as described in Section 6.11 by

transmitting a RERR message for that destination.

On the other hand, if the node receives one or more RREPs (or other

control message creating or updating the route to the desired

destination) during the discovery period, it first compares the hop

count of the new route with the value in the hop count field of the

invalid route table entry for that destination. If the hop count of

the newly determined route to the destination is greater than the hop

count of the previously known route the node SHOULD issue a RERR

message for the destination, with the 'N' bit set. Then it proceeds

as described in Section 6.7, updating its route table entry for that

destination.

A node that receives a RERR message with the 'N' flag set MUST NOT

delete the route to that destination. The only action taken should

be the retransmission of the message, if the RERR arrived from the

next hop along that route, and if there are one or more precursor

nodes for that route to the destination. When the originating node

receives a RERR message with the 'N' flag set, if this message came

from its next hop along its route to the destination then the

originating node MAY choose to reinitiate route discovery, as

described in Section 6.3.

Local repair of link breaks in routes sometimes results in increased

path lengths to those destinations. Repairing the link locally is

likely to increase the number of data packets that are able to be

delivered to the destinations, since data packets will not be dropped

as the RERR travels to the originating node. Sending a RERR to the

originating node after locally repairing the link break may allow the

originator to find a fresh route to the destination that is better,

based on current node positions. However, it does not require the

originating node to rebuild the route, as the originator may be done,

or nearly done, with the data session.

When a link breaks along an active route, there are often multiple

destinations that become unreachable. The node that is upstream of

the lost link tries an immediate local repair for only the one

destination towards which the data packet was traveling. Other

routes using the same link MUST be marked as invalid, but the node

handling the local repair MAY flag each such newly lost route as

locally repairable; this local repair flag in the route table MUST be

reset when the route times out (e.g., after the route has been not

been active for ACTIVE_ROUTE_TIMEOUT). Before the timeout occurs,

these other routes will be repaired as needed when packets arrive for

the other destinations. Hence, these routes are repaired as needed;

if a data packet does not arrive for the route, then that route will

not be repaired. Alternatively, depending upon local congestion, the

node MAY begin the process of establishing local repairs for the

other routes, without waiting for new packets to arrive. By

proactively repairing the routes that have broken due to the loss of

the link, incoming data packets for those routes will not be subject

to the delay of repairing the route and can be immediately forwarded.

However, repairing the route before a data packet is received for it

runs the risk of repairing routes that are no longer in use.

Therefore, depending upon the local traffic in the network and

whether congestion is being experienced, the node MAY elect to

proactively repair the routes before a data packet is received;

otherwise, it can wait until a data is received, and then commence

the repair of the route.

6.13. Actions After Reboot

A node participating in the ad hoc network must take certain actions

after reboot as it might lose all sequence number records for all

destinations, including its own sequence number. However, there may

be neighboring nodes that are using this node as an active next hop.

This can potentially create routing loops. To prevent this

possibility, each node on reboot waits for DELETE_PERIOD before

transmitting any route discovery messages. If the node receives a

RREQ, RREP, or RERR control packet, it SHOULD create route entries as

appropriate given the sequence number information in the control

packets, but MUST not forward any control packets. If the node

receives a data packet for some other destination, it SHOULD

broadcast a RERR as described in subsection 6.11 and MUST reset the

waiting timer to expire after current time plus DELETE_PERIOD.

It can be shown [4] that by the time the rebooted node comes out of

the waiting phase and becomes an active router again, none of its

neighbors will be using it as an active next hop any more. Its own

sequence number gets updated once it receives a RREQ from any other

node, as the RREQ always carries the maximum destination sequence

number seen en route. If no such RREQ arrives, the node MUST

initialize its own sequence number to zero.

6.14. Interfaces

Because AODV should operate smoothly over wired, as well as wireless,

networks, and because it is likely that AODV will also be used with

multiple wireless devices, the particular interface over which

packets arrive must be known to AODV whenever a packet is received.

This includes the reception of RREQ, RREP, and RERR messages.

Whenever a packet is received from a new neighbor, the interface on

which that packet was received is recorded into the route table entry

for that neighbor, along with all the other appropriate routing

information. Similarly, whenever a route to a new destination is

learned, the interface through which the destination can be reached

is also recorded into the destination's route table entry.

When multiple interfaces are available, a node retransmitting a RREQ

message rebroadcasts that message on all interfaces that have been

configured for operation in the ad-hoc network, except those on which

it is known that all of the nodes neighbors have already received the

RREQ For instance, for some broadcast media (e.g., Ethernet) it may

be presumed that all nodes on the same link receive a broadcast

message at the same time. When a node needs to transmit a RERR, it

SHOULD only transmit it on those interfaces that have neighboring

precursor nodes for that route.

7. AODV and Aggregated Networks

AODV has been designed for use by mobile nodes with IP addresses that

are not necessarily related to each other, to create an ad hoc

network. However, in some cases a collection of mobile nodes MAY

operate in a fixed relationship to each other and share a common

subnet prefix, moving together within an area where an ad hoc network

has formed. Call such a collection of nodes a "subnet". In this

case, it is possible for a single node within the subnet to advertise

reachability for all other nodes on the subnet, by responding with a

RREP message to any RREQ message requesting a route to any node with

the subnet routing prefix. Call the single node the "subnet router".

In order for a subnet router to operate the AODV protocol for the

whole subnet, it has to maintain a destination sequence number for

the entire subnet. In any such RREP message sent by the subnet

router, the Prefix Size field of the RREP message MUST be set to the

length of the subnet prefix. Other nodes sharing the subnet prefix

SHOULD NOT issue RREP messages, and SHOULD forward RREQ messages to

the subnet router.

The processing for RREPs that give routes to subnets (i.e., have

nonzero prefix length) is the same as processing for host-specific

RREP messages. Every node that receives the RREP with prefix size

information SHOULD create or update the route table entry for the

subnet, including the sequence number supplied by the subnet router,

and including the appropriate precursor information. Then, in the

future the node can use the information to avoid sending future RREQs

for other nodes on the same subnet.

When a node uses a subnet route it may be that a packet is routed to

an IP address on the subnet that is not assigned to any existing node

in the ad hoc network. When that happens, the subnet router MUST

return ICMP Host Unreachable message to the sending node. Upstream

nodes receiving such an ICMP message SHOULD record the information

that the particular IP address is unreachable, but MUST NOT

invalidate the route entry for any matching subnet prefix.

If several nodes in the subnet advertise reachability to the subnet

defined by the subnet prefix, the node with the lowest IP address is

elected to be the subnet router, and all other nodes MUST stop

advertising reachability.

The behavior of default routes (i.e., routes with routing prefix

length 0) is not defined in this specification. Selection of routes

sharing prefix bits should be according to longest match first.

8. Using AODV with Other Networks

In some configurations, an ad hoc network may be able to provide

connectivity between external routing domains that do not use AODV.

If the points of contact to the other networks can act as subnet

routers (see Section 7) for any relevant networks within the external

routing domains, then the ad hoc network can maintain connectivity to

the external routing domains. Indeed, the external routing networks

can use the ad hoc network defined by AODV as a transit network.

In order to provide this feature, a point of contact to an external

network (call it an Infrastructure Router) has to act as the subnet

router for every subnet of interest within the external network for

which the Infrastructure Router can provide reachability. This

includes the need for maintaining a destination sequence number for

that external subnet.

If multiple Infrastructure Routers offer reachability to the same

external subnet, those Infrastructure Routers have to cooperate (by

means outside the scope of this specification) to provide consistent

AODV semantics for ad hoc Access to those subnets.

9. Extensions

In this section, the format of extensions to the RREQ and RREP

messages is specified. All such extensions appear after the message

data, and have the following format:

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Length type-specific data ...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

where:

Type 1-255

Length The length of the type-specific data, not including the Type

and Length fields of the extension in bytes.

Extensions with types between 128 and 255 may NOT be skipped. The

rules for extensions will be spelled out more fully, and conform to

the rules for handling IPv6 options.

9.1. Hello Interval Extension Format

0 1 2 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type Length Hello Interval ...

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

... Hello Interval, continued

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type 1

Length 4

Hello Interval

The number of milliseconds between successive transmissions

of a Hello message.

The Hello Interval extension MAY be appended to a RREP message with

TTL == 1, to be used by a neighboring receiver in determine how long

to wait for subsequent such RREP messages (i.e., Hello messages; see

section 6.9).

10. Configuration Parameters

This section gives default values for some important parameters

associated with AODV protocol operations. A particular mobile node

may wish to change certain of the parameters, in particular the

NET_DIAMETER, MY_ROUTE_TIMEOUT, ALLOWED_HELLO_LOSS, RREQ_RETRIES, and

possibly the HELLO_INTERVAL. In the latter case, the node should

advertise the HELLO_INTERVAL in its Hello messages, by appending a

Hello Interval Extension to the RREP message. Choice of these

parameters may affect the performance of the protocol. Changing

NODE_TRAVERSAL_TIME also changes the node's estimate of the

NET_TRAVERSAL_TIME, and so can only be done with suitable knowledge

about the behavior of other nodes in the ad hoc network. The

configured value for MY_ROUTE_TIMEOUT MUST be at least 2 *

PATH_DISCOVERY_TIME.

Parameter Name Value

---------------------- -----

ACTIVE_ROUTE_TIMEOUT 3,000 Milliseconds

ALLOWED_HELLO_LOSS 2

BLACKLIST_TIMEOUT RREQ_RETRIES * NET_TRAVERSAL_TIME

DELETE_PERIOD see note below

HELLO_INTERVAL 1,000 Milliseconds

LOCAL_ADD_TTL 2

MAX_REPAIR_TTL 0.3 * NET_DIAMETER

MIN_REPAIR_TTL see note below

MY_ROUTE_TIMEOUT 2 * ACTIVE_ROUTE_TIMEOUT

NET_DIAMETER 35

NET_TRAVERSAL_TIME 2 * NODE_TRAVERSAL_TIME * NET_DIAMETER

NEXT_HOP_WAIT NODE_TRAVERSAL_TIME + 10

NODE_TRAVERSAL_TIME 40 milliseconds

PATH_DISCOVERY_TIME 2 * NET_TRAVERSAL_TIME

RERR_RATELIMIT 10

RING_TRAVERSAL_TIME 2 * NODE_TRAVERSAL_TIME *

(TTL_VALUE + TIMEOUT_BUFFER)

RREQ_RETRIES 2

RREQ_RATELIMIT 10

TIMEOUT_BUFFER 2

TTL_START 1

TTL_INCREMENT 2

TTL_THRESHOLD 7

TTL_VALUE see note below

The MIN_REPAIR_TTL should be the last known hop count to the

destination. If Hello messages are used, then the

ACTIVE_ROUTE_TIMEOUT parameter value MUST be more than the value

(ALLOWED_HELLO_LOSS * HELLO_INTERVAL). For a given

ACTIVE_ROUTE_TIMEOUT value, this may require some adjustment to the

value of the HELLO_INTERVAL, and consequently use of the Hello

Interval Extension in the Hello messages.

TTL_VALUE is the value of the TTL field in the IP header while the

expanding ring search is being performed. This is described further

in section 6.4. The TIMEOUT_BUFFER is configurable. Its purpose is

to provide a buffer for the timeout so that if the RREP is delayed

due to congestion, a timeout is less likely to occur while the RREP

is still en route back to the source. To omit this buffer, set

TIMEOUT_BUFFER = 0.

DELETE_PERIOD is intended to provide an upper bound on the time for

which an upstream node A can have a neighbor B as an active next hop

for destination D, while B has invalidated the route to D. Beyond

this time B can delete the (already invalidated) route to D. The

determination of the upper bound depends somewhat on the

characteristics of the underlying link layer. If Hello messages are

used to determine the continued availability of links to next hop

nodes, DELETE_PERIOD must be at least ALLOWED_HELLO_LOSS *

HELLO_INTERVAL. If the link layer feedback is used to detect loss of

link, DELETE_PERIOD must be at least ACTIVE_ROUTE_TIMEOUT. If hello

messages are received from a neighbor but data packets to that

neighbor are lost (e.g., due to temporary link asymmetry), we have to

make more concrete assumptions about the underlying link layer. We

assume that such asymmetry cannot persist beyond a certain time, say,

a multiple K of HELLO_INTERVAL. In other words, a node will

invariably receive at least one out of K subsequent Hello messages

from a neighbor if the link is working and the neighbor is sending no

other traffic. Covering all possibilities,

DELETE_PERIOD = K * max (ACTIVE_ROUTE_TIMEOUT, HELLO_INTERVAL)

(K = 5 is recommended).

NET_DIAMETER measures the maximum possible number of hops between two

nodes in the network. NODE_TRAVERSAL_TIME is a conservative estimate

of the average one hop traversal time for packets and should include

queuing delays, interrupt processing times and transfer times.

ACTIVE_ROUTE_TIMEOUT SHOULD be set to a longer value (at least 10,000

milliseconds) if link-layer indications are used to detect link

breakages such as in IEEE 802.11 [5] standard. TTL_START should be

set to at least 2 if Hello messages are used for local connectivity

information. Performance of the AODV protocol is sensitive to the

chosen values of these constants, which often depend on the

characteristics of the underlying link layer protocol, radio

technologies etc. BLACKLIST_TIMEOUT should be suitably increased if

an expanding ring search is used. In such cases, it should be

{[(TTL_THRESHOLD - TTL_START)/TTL_INCREMENT] + 1 + RREQ_RETRIES} *

NET_TRAVERSAL_TIME. This is to account for possible additional route

discovery attempts.

11. Security Considerations

Currently, AODV does not specify any special security measures. Route

protocols, however, are prime targets for impersonation attacks. In

networks where the node membership is not known, it is difficult to

determine the occurrence of impersonation attacks, and security

prevention techniques are difficult at best. However, when the

network membership is known and there is a danger of such attacks,

AODV control messages must be protected by use of authentication

techniques, such as those involving generation of unforgeable and

cryptographically strong message digests or digital signatures.

While AODV does not place restrictions on the authentication

mechanism used for this purpose, IPsec AH is an appropriate choice

for cases where the nodes share an appropriate security association

that enables the use of AH.

In particular, RREP messages SHOULD be authenticated to avoid

creation of spurious routes to a desired destination. Otherwise, an

attacker could masquerade as the desired destination, and maliciously

deny service to the destination and/or maliciously inspect and

consume traffic intended for delivery to the destination. RERR

messages, while less dangerous, SHOULD be authenticated in order to

prevent malicious nodes from disrupting valid routes between nodes

that are communication partners.

AODV does not make any assumption about the method by which addresses

are assigned to the mobile nodes, except that they are presumed to

have unique IP addresses. Therefore, no special consideration, other

than what is natural because of the general protocol specifications,

can be made about the applicability of IPsec authentication headers

or key exchange mechanisms. However, if the mobile nodes in the ad

hoc network have pre-established security associations, it is

presumed that the purposes for which the security associations are

created include that of authorizing the processing of AODV control

messages. Given this understanding, the mobile nodes should be able

to use the same authentication mechanisms based on their IP addresses

as they would have used otherwise.

12. IANA Considerations

AODV defines a "Type" field for messages sent to port 654. A new

registry has been created for the values for this Type field, and the

following values have been assigned:

Message Type Value

--------------------------- -----

Route Request (RREQ) 1

Route Reply (RREP) 2

Route Error (RERR) 3

Route-Reply Ack (RREP-ACK) 4

AODV control messages can have extensions. Currently, only one

extension is defined. A new registry has been created for the Type

field of the extensions:

Extension Type Value

--------------------------- -----

Hello Interval 1

Future values of the Message Type or Extension Type can be allocated

using standards action [2].

13. IPv6 Considerations

See [6] for detailed operation for IPv6. The only changes to the

protocol are that the address fields are enlarged.

14. Acknowledgments

Special thanks to Ian Chakeres, UCSB, for his extensive suggestions

and contributions to recent revisions.

We acknowledge with gratitude the work done at University of

Pennsylvania within Carl Gunter's group, as well as at Stanford and

CMU, to determine some conditions (especially involving reboots and

lost RERRs) under which previous versions of AODV could suffer from

routing loops. Contributors to those efforts include Karthikeyan

Bhargavan, Joshua Broch, Dave Maltz, Madanlal Musuvathi, and Davor

Obradovic. The idea of a DELETE_PERIOD, for which expired routes

(and, in particular, the sequence numbers) to a particular

destination must be maintained, was also suggested by them.

We also acknowledge the comments and improvements suggested by Sung-

Ju Lee (especially regarding local repair), Mahesh Marina, Erik

Nordstrom (who provided text for section 6.11), Yves Prelot, Marc

Mosko, Manel Guerrero Zapata, Philippe Jacquet, and Fred Baker.

15. Normative References

[1] Bradner, S. "Key words for use in RFCs to Indicate Requirement

Levels", BCP 14, RFC2119, March 1997.

[2] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA

Considerations Section in RFCs", BCP 26, RFC2434, October 1998.

16. Informative References

[3] Manner, J., et al., "Mobility Related Terminology", Work in

Progress, July 2001.

[4] Karthikeyan Bhargavan, Carl A. Gunter, and Davor Obradovic.

Fault Origin Adjudication. In Proceedings of the Workshop on

Formal Methods in Software Practice, Portland, OR, August 2000.

[5] IEEE 802.11 Committee, AlphaGraphics #35, 10201 N.35th Avenue,

Phoenix AZ 85051. Wireless LAN Medium Access Control MAC and

Physical Layer PHY Specifications, June 1997. IEEE Standard

802.11-97.

[6] Perkins, C., Royer, E. and S. Das, "Ad hoc on demand distance

vector (AODV) routing for ip version 6", Work in Progress.

17. Authors' Addresses

Charles E. Perkins

Communications Systems Laboratory

Nokia Research Center

313 Fairchild Drive

Mountain View, CA 94303

USA

Phone: +1 650 625 2986

Fax: +1 650 691 2170 (fax)

EMail: Charles.Perkins@nokia.com

Elizabeth M. Belding-Royer

Department of Computer Science

University of California, Santa Barbara

Santa Barbara, CA 93106

Phone: +1 805 893 3411

Fax: +1 805 893 8553

EMail: ebelding@cs.ucsb.edu

Samir R. Das

Department of Electrical and Computer Engineering

& Computer Science

University of Cincinnati

Cincinnati, OH 45221-0030

Phone: +1 513 556 2594

Fax: +1 513 556 7326

EMail: sdas@ececs.uc.edu

18. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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